I guess there is a hit triple: "The Verdict In The Story", "Wannabe In The Weeds" and "The Pain In The Heart". I can't decide, which is best, alltogether they make a stunning impression on me.
In my opinion, none of following episodes makes stronger impression.
Season 1-i think the very first episode..and the finale...
Season 2-"The woman in the sand" and "Two Bodies in the Lab"
Season 3-"The pain in the heart" and "the mummy in the maze"
Season 4-"Critic in the Cabernet"
Season 5-"The Harbinger in the fountain" "The Rocker in the Rinse cycle" "The parts in the sum of the whole" and "The End in the beginning"